Yes, the terms "riceboy" and "rice rocket" has some deep roots, and started out as a reference to a vehicle's country of origin, but also carried many of the negative connotations and stereotypes that the majority of Americans held for said country/culture.
As it became politically incorrect to hold such implications and stereotypes, people reworked these mild racial derogatives into facially neutral terms. Instead of un-PC references to any particular country or ethnicity, "rice" (despite the fact that this particular term is intimately linked with the American perception of Asian ethnicities) is now more-or-less defined as "a preoccupation with the facade of high performance." (The definition must not necessitate any association with aftermarket accessories, or otherwise it would have no application to the IS300, right?)
In light of the terms' racist origins, it's not surprising that the vast majority of the applications have been to Japanese vehicles and Asian car enthusiasts, even though the recently sanitized definition can be effectively applied to lowriders, hot rods (not all rods are fast, the style is simply looking optimized for drag), and cars such as the Chrysler Prowler, pseudo-Saleens, and all the tail-finned super-embellished cars from the 50's and 60's.
But when have you heard any of these cars or enthusiasts being called "riceboys" or "rice rockets"? Never.
Remember the term "facially neutral".
(Americans have had a great deal of experience in transforming essentially unsavory practices into facially neutral ones so that injustice can be given an excuse to exist. See slavery, arguments for; segregation, separate but "equal"; redlining, voting districts; sex discrimination, military; and ad infinitum.)